The present invention relates to a balancer structure for three-cylinder engines, and more particularly to a device provided with a countershaft rotated at the same speed but in the opposite direction of the crankshaft of the engine so as to balance the primary couple of inertia forces of the crankshaft about an intermediate position in the axial direction.
There are two inertia forces of reciprocating masses and rotating masses, causing vibrations in the engine. The inertia forces of rotating masses may be balanced by providing a counterweight on the crankshaft in the opposite direction to a crank arm. The inertia forces of reciprocating masses may be balanced by the counterweight by a half of the inertia forces and the remainder may be balanced by the countershaft which is rotated in the opposite direction from the crankshaft and at the same speed.
However, in the three-cylinder engine, the inertia forces of the first cylinder and the third cylinder act on the crankshaft symmetrically about an intermediate point corresponding to the second cylinder which is disposed between the first and third cylinders. Thus, an inertia couple acts about the intermediate point on the crankshaft. The couple of inertia causes a considerable vibration in the engine. Even if the inertia forces of rotating masses and reciprocating masses are balanced and further if the couple of inertia about the X-axis is balanced, the couple of inertia about an axis perpendicular to the crankshaft is inevitably generated. In order to balance such a couple of inertia, Japanese patent application laid open 55-6035 provides a balancer device of counterweights having a separated structure. Japanese patent publication 54-2333 discloses a countershaft which generates a couple of inertia equal to the couple of inertia of the crankshaft but opposite to the direction thereof.